At present, magnetic cards such as cash cards, credit cards and the like have been widely used, but recently there has been proposed an optical data recording card upon which is optically recorded information in order to improve recording density and to ensure reliable data storage. For instance, PCT/US 84/01463 (International Publication on Apr. 25, 1985: International Publication No. W085/01818) discloses a technique developed by DREXLER TECHNOLOGY CORPORATION in USA concerned with optical data recording cards.
However, in order to accurately read out stored data and to efficiently access data, data must be recorded based upon a predetermined format. In order to attain an optimum format, there still exist various problems which must be solved and so far there has not been proposed a practical optimum method for recording data onto optical data recording cards. Especially in order to ensure the readout of data and the efficient access to data, the following practical problems constitute a barrier:
(Problem 1: control of a line sensor for reading out a unit readout array)
A line sensor reads out the data recorded in the form of pits from respective unit readout arrays and in order to properly shift the line sensor to a predetermined position for reading out the data stored in a specified unit readout array, a suitable data recording format must be devised. Especially, the control must be carried out accurately so that the direction of rows of the unit readout arrays must coincide with the direction of the sensor array of the line sensor with a high degree of accuracy
(Problem 2: synchronization with data pits)
The data pits are recorded and spaced apart from each other by a predetermined distance. In case of reading out such data pits by the line sensor, the readout timing must be in synchronism with the distance between the pits. Therefore, a data recording format must be provided with a means for attaining such synchronization.
(Problem 3: discrimination between the adjacent unit readout arrays)
The line sensor reads out the data pits in respective unit readout arrays in the manner described above and when the data from one unit readout array is read out, the operation for reading out the data stored in the next unit readout array is executed. In this manner, a plurality of unit readout arrays are read out sequentially, but in case of scanning the line sensor, it must be detected that the operation for reading out the next readout array has been started. That is, a data recording format bearing the information capable of distinguishing between the adjacent unit readout arrays must be devised.
(Problem 4: countermeasures against readout errors)
In general, since individual person carries and uses the optical data recording cards, the data recording surfaces tend to be contaminated. Optical reading systems are very sensitive to such surface contamination, resulting in readout errors. It follows therefore that there must be devised a data recording format capable of avoiding such readout errors.
(Problem 5: efficient access to data)
As described above, since the optical data cards are carried by individual persons, their dimensions or sizes must be so selected that they may be put into pockets. Meanwhile, various data must be recorded in a limited space, therefore a data recording format capable of permitting the easy access to the stored data must be devised.